Ion mobility detectors are the primary instruments used in the field of plasma chromatography. Generally, the operation of an ion mobility detector is similar to the operation of a time of flight mass spectrometer, the obvious difference being that a time of flight mass spectrometer operates in a vacuum where the mean free path of the contained gases is many times the dimensions of the gas container, while the ion mobility detector operates generally at atmospheric pressure where the mean free path of the contained gases is a small fraction of the dimensions of the container. More particularly, a typical ion mobility detector is comprised of a combined ionization source and an ion reaction region, an ion drift region and an ion injection shutter or grid interposed between the ion reaction region and the ion drift region. A carrier gas, normally purified air or nitrogen, is introduced into the ion mobility detector to transport sample vapor of a material, whose identity is to be characterized, into the ion mobility detector, so that the gaseous mixture is exposed to the ionization source. As a result, portions of both the carrier gas and the sample are directly ionized by the ionization source. However, as known to those practicing in this art, the characteristics of the carrier gas and the sample are usually such that the molecules of the carrier gas are more easily directly ionized by the ionization source than are the molecules of the sample. At this time the gaseous mixture is contained within the reaction region. Since the mean free path is many times smaller than the dimensions of the reaction region there are multiple collisions between the molecules of the carrier and sample gases. As also known to those skilled in the art, the tendency of these collisions is to transfer the ion charge from the carrier molecules to the sample molecules, thereby ionizing the sample gas mainly by this secondary ionization process.
The charged particles or ions, derived from both the sample and carrier gas, are accelerated to a terminal velocity under the influence of a field potential gradient within the reaction region toward an ion injection grid which, as mentioned earlier, separates the reaction region from the drift region. The grid is normally electrically biased to prevent the transfer of ions from the reaction region to the drift region. Periodically, the grid is deenergized for a short time period to permit a pulse of ions to pass therethrough into the drift region. Here, the ions, under the influence of an electrostatic drift field are drawn to an electrometer detector which terminates the drift region. The time of arrival of each ion at the electrometer detector, relative to the time the grid was opened, is determined by the ion's mobility through the non-ionized gas occupying the drift region. The heavier ions characteristically move more slowly through the drift region and arrive at the electrometer detector after longer drift times than lighter ions. It is thus possible to characterize the ions and hence, the sample by observing the time between the opening of the grid and the arrival of ions at the electrometer detector.
In a practical sense, an ion mobility detector may be used to determine whether a certain sample is present in an environment, such as a certain contaminant in atmospheric air. In this case the electrometer detector is sampled at predetermined times after the grid is opened to discover whether pulses of ions are then arriving at the electrometer detector. If electric current is measured then it can be concluded that the contaminant is present.
In the prior art, as mentioned above, the gaseous or vaporous sample, whose identity is to be characterized by the ion mobility detector, is injected or drawn into the reaction region to react with carrier gas ions formed therein by the ionization source. If it is desired to determine whether the atmosphere contains a certain component, usually a contaminant, the sample can simply be a sample of ambient air.
Several problems are encountered when using ion mobility detectors for environmental sampling purposes. A first problem involves no alarms due to interferences from the normal composition (e.g. oxygen, water, ammonia and/or nitrogen oxides) of the ambient air being drawn into the reaction region of the detector cell. The second involves false alarms or no alarms due to interferences from extraneous vapor components contained in the ambient air being drawn into the reactor region of the detector cell.
The first problem is associated with the principles underlying the tendency of a charge residing on a reactant ion to transfer to a neutral sample molecule. The transfer of the charge is necessary if a produce ion is to be formed from the sample molecule and the sample molecules are to be detected. As is known to those skilled in the art, this tendency to transfer charge is related to the relative proton and/or electron affinities of the ions and molecules present in the reactor region either due to composition of the carrier gas or to products of the ionization process. Since ammonia has a relatively high proton affinity and the oxides of nitrogen have high electron affinities, very few sample molecules can remove charge from these normal components of ambient air for ionization purposes. Examples include the inability to detect halogenated compounds in the presence of negative ions of nitrogen dioxide and to detect acids and alcohols in the presence of the positive ammonium ion. Hence, to allow ambient air sampling by an ion mobility detector, a means must be provided that will discriminate between the entrance of these normal components of ambient air and the sample molecules of interest into the ion mobility detector.
The second problem is associated not only with the principles underlying the tendency of a charge residing on a reactant ion to transfer to a neutral sample molecule, but also the probabilities associated with extraneous component vapors contained in ambient air having similar ion mobilities to the sample compound of interest. That is, if the environmental sample contains one or more extraneous components whose ion charge is the same as, and whose ion mobility is similar to, that of the sample it is desired to detect, then ions of the extraneous components will arrive at the electrometer detector at such a drift time as to indicate the looked for sample is present when in fact it may not be, thus causing a false indication or alarm. An aggravated example of this problem is the tendency of the normal alkanes to cluster into molecules or ions of larger mass and/or decompose into ions of lower mass so that the electrometer detector senses the arrival of ions with a wide range of drift times. That is, irrespective of the drift time for the sample molecule, an ion is formed from the normal alkanes whose mobility approximates that of the looked for sample molecule. This leads to a false indication or alarm as described above. Since a problem of this type is most severe for interferants at high concentration, a means must be provided that will discriminate between the entrance of these problem interferants and the sample molecules of interest into the ion mobility detector.